An expected yield of corn is often estimated in bushels-per-acre; such estimates are often made by examining a small number of corn ears believed to be representative. Crop scouts typically estimate the number of kernels in 1/1000 of an acre by estimating the average number of kernels per ear, then multiplying by the number of ears in 1/1000 of an acre. To estimate the average number of kernels per ear, the number of kernel rows and average row length in kernels is averaged for several representative ears. The number of kernels per 1/1000 of an acre is then obtained by multiplying the average number of kernels-per-ear by the number of ears counted in 1/1000 of an acre. Finally, the number of kernels per 1/1000 of an acre is then divided by an assumed number of kernels per bushel to get an estimated number of bushels-per 1/1000 of an acre, which can be easily expressed as bushels per acre.
Such estimates are often inaccurate due to reliance on an assumed conversion constant of kernels-per-bushel. Some literature suggests using 90,000 kernels-per-bushel, but modern hybrids may have as few as 70,000 kernels-per-bushel due to larger kernel size. Using any fixed conversion factor leads to potential error. Other sources recommend estimating the number of kernels-per-bushel based on growing conditions, but this is circular logic since the point of doing the point yield estimate is to determine how well the corn is growing.
A crop farmer's livelihood is dependent upon yield, so accurate estimates are crucial for planning purposes. The impact of kernel size on yield estimates can be striking: a difference of only 0.06 grams per kernel can lead to a more than 20% swing in yield estimates.
There are two widely used methods for point yield prior to harvesting: the Yield Component Method, and the Pro Farmer Midwest Crop Tour Method. The Yield Component Method requires estimating the kernel-per-bushel based on whether growing conditions are “excellent,” “average,” or “poor.” This method has drawbacks; for one, it employs somewhat circular logic, since the purpose of estimating the point yield is to determine how well the corn is growing—i.e., how favorable the growing conditions are. Characterizing growing conditions is also an inherently subjective exercise that introduces a potential bias into the estimate.
The Pro Farmer Midwest Crop Tour Method uses the length of the ear in inches as a proxy for the number of kernels in a row, and assumes the kernels-per-bushel (in thousands) is 0.345 of the row length in kernels. Yet potential ear length will vary dramatically from year to year as growing conditions vary. Stress can greatly reduce potential kernel number per row. Conversely, excellent growing conditions can encourage development of an unusually high number of kernels per ear. While the length of the ear is an important indicator, therefore, it can lead to inaccurate estimates.